This invention relates to pump jacks used in the construction trade to erect elevated work platforms and, more particularly, to an improvement to the pump jack apparatus providing a safer means of lowering the work platform.
Scaffolding equipment is required at many stages in the construction or renovation of a building. In the construction trade, pump jack poles that are braced to a vertical surface and pump jacks placed thereon, are a well known alternative to complex scaffolding. The pump jacks support a work platform on which workmen can stand. A pump arm, actuated by the worker's foot, enables the worker to raise the platform up the poles to the required height. Disengaging a portion of the pump jack from the pole allows the workmen to lower the platform back to ground level.
Typically, at least two poles are placed in spaced apart relationship to each other and are secured by braces to the vertical side of a building. Pump jacks are then mounted on each pole. Each pump jack includes a platform support upon which the end of a wooden or metal plank is supported, thus forming a platform. The platform may be provided with an extension so as to allow it to accommodate two or more planks. A worker stands on the platform and raises the jack and the adjacent end of the platform by pumping the jack's pump arm with his foot. The pumping action raises the jack and the adjacent end of the platform in a stepwise manner up the pole. To lower the platform, the worker disengages one shackle (typically the "lower" shackle) on the jack while turning a crank actuated spiral rod that causes a second shackle (typically the "upper" shackle) to creep down the pole and thereby lower the platform.
Pump jacks are well known. For example, U.S. Pat. Nos. 1,416,296, 1,441,806, 2,038,899, and 2,216,912 all issued to P. Hoitsma and U.S. Pat. No. 4,463,828 issued to C. Anderson, teach pump jacks of the type contemplated by the instant invention.
In a typical prior art pump jack a metal frame supports a lower metal shackle assembly. The lower shackle assembly includes front and rear clamping bars that apply a coupling force for gripping the pole. A pump arm is pivotally coupled to the frame and an upper shackle assembly that is generally similar in construction to the lower shackle assembly. The upper shackle assembly includes a front clamping member in the form of a rotatable spiral rod and a rear clamping bar. The rear clamping bar of the upper shackle assembly applies a coupling force for gripping the pole. Biasing mechanisms forming part of the shackle assemblies respond to the pumping action of the pump arm to alternately position each of the shackles in gripping relationship with the pole. The pump arm operates in a downward stroke to pivotally drive the frame stepwise upwardly along the pole, with the upper shackle gripping the pole and providing the upward moving force. When the pump arm is moved upwardly by the operator's foot, the upper shackle moves upwardly along the pole while the lower shackle grips the pole and the frame remains stationary. A single workman can lift the platform up the poles to the required height by alternately pumping the pump arms of each pump jack. Alternatively, two workmen can work in unison to pump the platform up the poles. Lowering the platform can be achieved by a single workman operating each jack in alternating fashion, or by two workmen operating the two jacks in unison with one another.
In order to bring the pump jack down the pole, the lower shackle must be released from gripping relationship with the pole. Typically, the lower shackle is moved by the operator so that its front clamping bar is no longer gripping the surface of the pole. A crank arm on the end of the spiral rod, allows the workman to crank the pump jack down the pole while the lower shackle is disengaged (released) from the pole.
In pump jacks preceding the Anderson invention, releasing of the lower shackles to permit platform lowering was accomplished by having the workman or operator extend one of his feet out and around the pole toward the front of the pump jack into engagement with the front clamping bar of the lower shackle. That foot then was used to force the shackle in a direction to cause its front clamping bar to release the pole. Such pump jacks can be lowered only while the front clamping bar of the lower shackle is displaced by the pressure of the worker's foot far enough to release its grip on the pole. If the worker's foot misses the end of the clamping bar or slips off, the worker can lose his balance and fall off the platform. This situation creates a major safety hazard, especially when the platform is raised to heights in excess of 20-30 feet above the ground. Worker's safety regulations in the U.S., Canada, and Europe have mandated scaffolding that does not require the worker to extend any part of his body beyond the platform in order to maneuver the scaffolding.
In the patent to Anderson mentioned above, a pedal-type release mechanism has been incorporated into the pump jack to release the lower shackle. The pedal-type release mechanism is disposed on one side of the frame and comprises a leg having one end coupled to the lower shackle. This leg is slidably retained by a clamp located on the outside of the frame. The release mechanism includes a foot-engaging pedal that is angled toward the adjacent frame member so as to bias a workmen's foot toward the frame of the pump jack.
Unfortunately, Anderson's invention suffers from several disadvantages. First, his design still requires an operator to extend his foot over one edge of the platform. Second, the release mechanism applies force to only one end of the front clamping bar of the lower shackle. This configuration applies a torsional force to the lower shackle that conceivably could impede satisfactory operation of the release mechanism and the desired movement of the shackle, possibly even causing a jam. If such jamming should occur, the front clamping bar may need to be kicked by the worker until it disengages the pole. Kicking the front clamp bar increases the likelihood that the worker will lose his balance and fall from the platform. Essentially, Anderson's design does not allow for a safe release of the front clamping bar.